In fuel cell systems capable of high-efficiency, small-scale power generation, it is easy to construct a system for utilizing heat energy generated during power generation. Thanks to this, fuel cell systems have heretofore been developed as a dispersed power generation system capable of achievement of high energy utilization efficiency.
Fuel cell systems have fuel cell as the main body of the power generation section. The fuel cell directly converts the chemical energy of fuel gas and oxidizing gas into electric energy through a predetermined electrochemical reaction. Therefore, in the fuel cell systems, the fuel gas and oxidizing gas are respectively supplied to the fuel cell during power generating operation. Then, in the fuel cell, the specified electrochemical reaction which uses the supplied fuel gas and oxidizing gas proceeds, thereby generating electric energy. The electric energy generated in the fuel cell is fed to loads from the fuel cell system. The fuel cell systems discussed herein generally include a reformer and a blower. In the reformer, the hydrogen-rich fuel gas is generated by the steam reforming reaction which uses water and a raw material containing an organic compound composed of at least carbon and hydrogen, which is a natural gas. This fuel gas is supplied to the fuel cell as a fuel for power generation. It should be noted that the steam reforming reaction proceeds with the reforming catalyst of the reformer being heated by, for example, a combustion burner. The blower suctions air from the atmosphere. This air is fed to the fuel cell as the oxidizing gas used for power generation.
In a heretofore known fuel cell system, when stopping a power generating operation, the supply of the raw material to the reformer is cut off. This stops the supply of the fuel gas from the reformer to the fuel cell, stopping the progress of the electrochemical reaction in the fuel cell. As a result, the supply of electric power from the fuel cell system to the loads is stopped. If the supply of the raw material to the reformer is cut off, the fuel gas, which has been generated before the cutoff of the raw material, will stagnate in the fuel cell and its surrounding area throughout the period during which the power generating operation is stopped. In this case, if the stagnant fuel gas is mixed with air coming from the combustion burner opened to the atmosphere owing to natural convection, hydrogen contained in the fuel gas will be rapidly oxidized by oxygen contained in the air, which gives rise to a risk that the fuel cell system may be damaged by the reaction heat generated by the oxidation reaction.
To prevent the stagnation of the fuel gas within the fuel cell system, the known fuel cell system is configured to feed inert gas such as nitrogen gas to the passage where the fuel gas is stagnant during a power generation stop period and to burn the fuel gas forced out of the passage in a combustion burner. According to this configuration, the stagnation of the fuel gas within the fuel cell etc. during a power generation stop period can be prevented and therefore the rapid oxidation of hydrogen contained in the fuel gas can be prevented so that the fuel cell system ensures high security.
This known fuel system, however, requires an inert gas feeding means such as a nitrogen cylinder installed within or near the fuel cell system in order to replace the stagnant fuel gas with the inert gas such as nitrogen gas. This makes the fuel cell system large-sized and therefore makes it difficult to use the fuel cell system as a stationary dispersed power generation system for household use or an electrical vehicular power plant. Furthermore, the use of the feeing means of the inert gas such as nitrogen gas in addition to the existing components increases the initial cost of the fuel cell system. In addition, the known fuel cell system needs periodical replacement or replenishment of the inert gas feeding means such as a nitrogen cylinder, which leads to an increase in the running cost of the fuel cell system.
In this known fuel cell system, just after the start of a power generating operation, the fuel gas containing a high concentration of carbon monoxide is supplied from the reformer to the fuel cell. The reason for this is that the operating temperature of the reformer has not reached a specified value at the start of a power generating operation and therefore a sufficient amount of carbon monoxide cannot be removed from the fuel gas. If a solid polymer electrolyte fuel cell, for example, is supplied with the fuel gas containing a high concentration of carbon monoxide, the catalyst of the fuel electrode of the solid polymer electrolyte fuel cell will be poisoned by the carbon monoxide supplied. The poisoning of the catalyst of the fuel electrode significantly hinders the progress of the electrochemical reaction in the fuel cell. Therefore, the known fuel cell system suffers from the problem that the power generating performance of the fuel cell deteriorates, depending upon the number of stops and starts of power generating operation.
As a fuel cell system that can be easily used in the home and electric cars and is unlikely to cause progressive poisoning of the catalyst, there has been proposed a fuel system that is configured to cut off the supply of the fuel gas to the fuel cell just after the start of a power generating operation and to introduce, as a substitution gas, the raw material of the fuel gas into the fuel cell after stopping the power generating operation (e.g., Patent Document 1).
The proposed fuel cell system is comprised of: a reformer for generating a hydrogen-rich fuel gas from a raw material containing as a chief component an organic compound that contains carbon and hydrogen; a fuel gas feed passage for feeding a fuel gas from the reformer to a fuel cell; an off gas feed passage for feeding the fuel gas, which has been discharged from the fuel cell without being used in power generation (hereinafter referred to as “off gas”), to a combustion burner of the reformer; and a first bypass route provided between the fuel gas feed passage and the off gas feed passage, for switching the destination of the fuel gas from the fuel cell to the combustion burner of the reformer. This fuel cell system also comprises a raw material feeder for feeding the raw material of the fuel gas to the reformer and a second bypass route for feeding the raw material directly from the raw material feeder to the fuel cell, so as to bypass the reformer.
In the proposed fuel cell system, just after the start of a power generating operation, the fuel gas containing a high concentration of carbon monoxide and generated in the reformer is supplied to the combustion burner of the reformer by way of the first bypass route. In the combustion burner, the fuel gas is combusted to heat the reforming catalyst. After the temperature of the reforming catalyst in the reformer has reached a specified value after the start of a power generating operation, the fuel gas generated in the reformer is supplied to the fuel cell by way of the fuel gas feed passage. Then, the fuel gas is used as a fuel for power generation in the fuel cell. The off gas discharged from the fuel cell is supplied to the combustion burner of the reformer by way of the off gas feed passage. Then, the off gas is combusted for heating the reforming catalyst in the combustion burner.
In the proposed fuel cell system, after stopping the power generating operation, the raw material serving as a substitution gas is introduced from the raw material feeder to a fuel gas flow path of the fuel cell by way of the second bypass route. Thereby, the inside of the fuel cell and its surrounding area are sealed up by the raw material instead of the inert gas such as nitrogen gas throughout the period during which the power generating operation of the fuel cell system is stopped.
According to this fuel cell system, the raw material serving as a substitution gas is introduced into the fuel cell from the conventionally-used raw material feeder after stopping the power generation, which eliminates the need for provision of an inert gas feeding means such as a nitrogen cylinder within or in the proximity of the fuel cell system. The fuel cell system can avoid growing in size and therefore can be used as a stationary dispersed power generation system for household use or an electrical vehicular power plant. In addition, since there is no need to provide an inert gas (nitrogen gas) feeding means in addition to existing components, the initial cost of the fuel cell system can be kept low. Furthermore, periodical replacement of an inert gas (nitrogen gas) feeding means is unnecessary, which makes it possible to keep the running cost of the fuel cell system low.
The material introduced from the raw material feeder into the fuel cell is chemically stable compared to hydrogen contained in the fuel gas. Therefore, even if the raw material, stagnating in the fuel cell throughout a power generating operation stop period, is mixed with air penetrating thereinto, no rapid oxidizing reaction will occur. Accordingly, damage to the fuel cell system caused by reaction heat generated in the oxidizing reaction can be effectively prevented, by introducing the raw material into the fuel cell. In this way, the fuel cell system can ensure security in the power generating operation stop period.
In the proposed fuel cell system, the fuel gas containing a high concentration of carbon monoxide is not supplied to the fuel cell just after the start of a power generating operation. After the temperature of the reforming catalyst in the reformer has reached the specified temperature and the fuel gas whose carbon monoxide concentration has been sufficiently reduced is generated, the fuel gas is supplied from the reformer to the fuel gas. This prevents the poisoning of the catalyst of the fuel electrode in the solid polymer electrolyte fuel cell. Thus, the factors that interrupt the progress of the electrochemical reaction in the fuel cell are swept away and therefore the problem that the power generating performance of the fuel cell deteriorates depending on the number of stops and starts of power generating operation can be solved.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2003-229149